Project server update task custom field using ReST API

Hello Everyone,

I'm trying to update the task custom field using ReST API. I've followed the steps mentioned here: 

https://powerusers.microsoft.com/t5/Connecting-To-Data/Updating-Task-Custom-Fields-in-Project-Online/td-p/322559 

But when I try to execute it, I get a 404 Not Found error. Does anyone have any idea about this or the correct way to do this?

Thanks & Regards,

Rohan